1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal display devices implenting in-plane switching (IPS) where an electric field to be applied to liquid crystals is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The long thin shapes of the liquid crystal can be aligned to have an orientation in a specific direction. The alignment direction of the liquid crystals can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy of the liquid crystal, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an electric field applied to a group of liquid crystal molecules in respective pixels, a desired image can be produced by diffracting light.
There are many types liquid crystal displays (LCDs). One type of LCD is an active matrix LCD (AM-LCD) that has a matrix of pixels. Each of the pixels in an AM-LCD has a thin film transistor (TFT) and a pixel electrode. AM-LCDs are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide applications in office automation (OA) equipment and video units because they have the characteristics of light weight, thin profile and low power consumption. The typical liquid crystal display panel of an LCD device has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors, and pixel electrodes.
The operation of an LCD device is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. More particularly, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of liquid crystal molecules is properly controlled in each pixel of a group of pixels, incident light is refracted along the alignment direction in a plurality of pixels to display image data. Thus, liquid crystal molecules in the pixels function as an optical modulation element having variable optical characteristics that depend upon the polarity of the applied voltage.
FIG. 1 is a partial perspective view illustrating a related art active matrix LCD device. As shown in FIG. 1, the LCD device includes an upper substrate 10 and a lower substrate 30 that are spaced apart from each other, and a liquid crystal layer 50 is interposed therebetween. The upper substrate 10 and the lower substrate 30 are often referred to as a color filter substrate and an array substrate, respectively. A common electrode 22 and a pixel electrode 24 are located on the lower substrate 20. On the lower substrate 30, a plurality of gate lines 32 and data lines 34 are perpendicularly positioned with respect to each other such that pairs of the crossing gate lines 32 and data lines 34 define pixel regions P. The thin film transistors T are formed adjacent to a corner in each of the respective pixel regions P. A pixel electrode 46 is formed in each of the respective pixel regions that is electrically connected to the thin film transistor T of the pixel region. Although not shown in FIG. 1, each thin film transistor T includes a gate electrode for receiving signal voltages from the gate line 32, a source electrode for receiving a data voltage from the data line 34, a drain electrode for transferring the data voltage to the pixel electrode 46, and a channel that can be turned-on and turned-off via a voltage applied to gate electrode. The pixel electrode 46 is usually formed of a transparent conductive material, such as indium tin oxide (ITO) or indium zinc oxide (IZO).
The upper substrate 10 includes a color filter layer 12 and a common electrode that are sequentially formed on the inner surface of the upper substrate 10 that faces the lower substrate 30. Although not shown in FIG. 12, the color filter layer 12 includes color filters that transmit corresponding wavelengths of light and a black matrix shielding the light in the color filter borders. The common electrode 16 is usually formed of a transparent conductive material, such as indium tin oxide (ITO) or indium zinc oxide (IZO).
On the outer surfaces of the upper substrate 10 and the lower substrate 30, an upper polarizer 52 and a lower polarizer 54 are respectively formed thereon. A backlight generating light is disposed underneath the lower polarizer 54.
The above-mentioned active matrix liquid crystal display device is commonly formed throughout a liquid crystal cell process that includes an array fabrication, a color filter fabrication and a liquid crystal injection between the array and color filter substrates. The array fabricating process includes forming the switching elements, e.g., the thin film transistors, and forming the pixel electrodes. The color filter fabricating process includes forming the color filter layer and forming the common electrode. Additionally, the liquid crystal injection process includes a panel fabrication process for attaching the color filter substrate to the array substrate and injecting the liquid crystals into a gap between the color filter substrate and the array substrate.
The panel fabrication process is much simpler than the array substrate and color filter substrate fabricating process because it does not require repeated patterning processes. Further, the panel fabrication process includes forming alignment layers on the inner surfaces of the substrates, forming a cell gap, cutting the substrates into a cell, and injecting liquid crystals. Accordingly, the liquid crystal cell process finally forms a liquid crystal panel for a liquid crystal display.
FIG. 2 is a plan view illustrating a liquid crystal cell arrangement on a bare glass. As shown, a cell area IIa and a dummy area IIb are defined on a bare glass 60. The cell area IIa is surrounded by the dummy area IIb, and it becomes a liquid crystal cell after the cutting process. The cell area IIa is divided into a first area 62 and a second area 64, as shown in FIG. 2, and the second area 64 is disposed in peripheries of the cell area IIa. Although not shown in FIG. 2, a plurality of array elements are formed in the first area 62. During the cutting process after injecting the liquid crystals in the cell area IIa, the bare glass 60 is cropped to remove the dummy cell area IIb. Although only one cell area is shown in FIG. 2, a lot of cell areas can be disposed on the bare glass 60 with an adequate arrangement. Further, the bare glass can be formed of plastic and acts as a base substrate in the liquid crystal panel.
The liquid crystal cells can be adopted in various kind of display devices, and the size of the liquid crystal cell can be various depending on the display device. Therefore, it is very important to properly arrange the liquid crystal cells on the bare glass in order to reduce the dummy area. If the dummy area increases, it will be waste of cost and material.
To overcome the above disadvantage, an MMG (Multi-Model on Glass) type arrangement, which efficiently arranges large-sized liquid crystal cells and small-sized liquid crystal cells on the bare glass, is adopted to effectively utilize the bare glass space. FIG. 3 is a diagram illustrating an MMG type arrangement of the liquid crystal cells for use in fabricating a TN (twisted nematic) mode liquid crystal display device according to a related art.
As shown in FIG. 3, a plurality of first cell areas IIIa and a plurality of second cell areas IIIb are disposed on a bare glass 66 with being spaced apart from one another. Each of the first cell areas IIIa has a first size, while each of the second cell areas IIIb has a second size. When arranging the first and second cell areas IIIa and IIIb on the bare glass 66, it is very important to utilize the space effectively.
In the TN mode liquid crystal display device, a rubbing process is applied to the bare glass 66 for the purpose of inducing the initial arrangement of liquid crystals. At this time, the rubbing directions respectively applied to the upper and lower substrates are perpendicular to each other. Namely, a first rubbing direction r1 applied to the lower bare glass is perpendicular to a second rubbing direction r2 applied to the upper bare glass, as shown in FIG. 3. The first rubbing direction r1 is substantially 45 degrees and is adopted in the lower substrate of the liquid crystal panel. And the second rubbing direction r2 is substantially 135 degrees and is adopted in the upper substrate of the liquid crystal panel.
The viewing angle properties are determined depending on the alignment direction of the TN liquid crystals. Since the rubbing process is applied to the bare glass, the liquid crystal cells on the bare glass have the same rubbing direction. However, since the liquid crystal cell areas are arranged longitudinally to let the widths be parallel when the liquid crystal cells having different sizes, for example, the first and second liquid crystal cell areas IIIa and IIIb, are disposed on the same bare glass, there may be some limitation in arranging liquid crystal cells on the bare glass or in utilizing the space of the bare glass efficiently.
In a conventional LCD device, since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to both the lower and upper substrates. However, the conventional LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to FIG. 1.
FIG. 4 is a cross-sectional view illustrating the concept of a related art IPS-LCD panel. As shown in FIG. 4, upper and lower substrates 80 and 70 are spaced apart from each other, and a liquid crystal layer 90 is interposed therebetween. The upper substrate 80 and the lower substrate 70 are often referred to as an array substrate and a color filter substrate, respectively. On the lower substrate 70, a common electrode 72 and the pixel electrode 74 are positioned to be spaced apart from one another. The common electrode 72 and the pixel electrode 74 are parallel to each other. On the surface of the upper substrate 80, a color filter layer (not shown) is commonly positioned in a position between the pixel electrode 74 and the common electrode 72 of the lower substrate 70. A voltage applied across the common electrode 72 and the pixel electrode 74 produces an electric field E through the liquid crystal layer 90. Liquid crystals 92 have a positive dielectric anisotropy, and thus they align so as to be parallel to the electric field E. The result is a wide viewing angle that ranges from about 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD panel, for example.
FIG. 5A is a plan view illustrating one pixel of an array substrate where straight pixel and common electrodes are disposed according to a related art IPS-LCD device, and FIG. 5B is a plan view illustrating one pixel of an array substrate where zigzag pixel and common electrodes are disposed according to another related are IPS-LCD device. As shown in FIGS. 5A and 5B, gate lines GL are transversely arranged across the figures and data lines DL are disposed substantially perpendicular to the gate lines GL. Common line CL runs in parallel with the gate lines GL and are spaced apart from each of the gate lines GL. The gate lines GL, the common line CL and a pair of the data lines DL define a pixel region P on the array substrate. A thin film transistor (TFT) T is disposed adjacent a corner of the pixel region P where one of the gate lines GL and one of the data lines cross.
Referring to FIG. 5A, three common electrodes 94 extend perpendicularly from the common line CL in each pixel region P. Among the three common electrodes 94, two common electrodes 94 are disposed next to the data lines DL, respectively. A pixel connecting line 95 is disposed next to the gate line GL on the side of the pixel P opposite to the common line CL. The pixel connecting line 95 is electrically connected to the TFT T parallel with the gate line GL. Pixel electrodes 96 extend perpendicularly from the pixel connecting line 95. Each of the pixel electrodes 96 is disposed between two of the common electrodes 94 and are parallel with the data line DL. Each of areas “AA” between one of the respective common electrodes 94 and one of the respective pixel electrodes 96 is defined as a block where the liquid crystal molecules are re-arranged by electric fields. In FIG. 5A, there are four blocks in one pixel region P. The area “AA” is often referred to as an aperture area.
As shown in FIG. 5B, the common and pixel electrodes are shaped in zigzag to accomplish multiple domains along the length of the electrodes. Some detailed explanations, especially those previously explained in reference to FIG. 5A, will be omitted with regard to FIG. 5B to prevent duplicate explanations. A pixel connecting line PL is disposed over a common line CL in FIG. 5B. Common electrodes 97 and pixel electrodes 98 are extended from the common and pixel connecting lines CL and PL, respectively, in an up-and-down direction. Both the common electrodes 97 and the pixel electrodes 98 have a zigzag shape with plural bent portions that alternate with each other. However, corresponding portions of the common electrodes 97 and the pixel electrodes 98 are parallel to each other. The zigzag shape defines the multiple domains in the pixel regions that are symmetrical to the bent portions of the common electrodes 97 and the pixel electrode 98. This zigzag shape with multiple domains further improves the viewing angle as compared to the straight shape shown in FIG. 5A. Also in FIG. 5B, each of areas “AA” defined between the respective common electrodes 97 and the respective pixel electrodes 98 can be denoted as a block where the liquid crystal molecules are re-arranged by the electric fields. In FIG. 5B, there are also four blocks in one pixel region P.
As shown in FIGS. 5A and 5B, the IPS-LCD devices according to the related arts re-arrange and orient the liquid crystal molecules using electric fields that are parallel with the array substrate. Thus, they can provide a wide viewing angle as opposed to an LCD device that uses electric fields perpendicular to the array substrate.
Rubbing processes are applied to the above-mentioned array substrates to induce an initial orientation of the liquid crystal molecules. As shown in FIG. 5A, the rubbing process is performed along a rubbing direction “RD” that forms a certain angle with the straight common electrodes 94 and pixel electrodes 96. The reason for the inclined rubbing direction with respect to the common electrodes 94 and pixel electrodes is to obtain a fast re-arrangement of the liquid crystals in correspondence with the electric field. In FIG. 5B, a rubbing direction “RD” is parallel with the data lines DL because the common electrodes 97 and the pixel electrodes 98 have a zigzag shape.
FIG. 6 is a diagram illustrating an arrangement of liquid crystal cells for use in fabricating an IPS-LCE device in accordance with the related art. As shown in FIG. 6, the rubbing direction is determined depending on whether the IPS-LCD has the straight or zigzag-shaped electrodes. As illustrated with reference to FIGS. 5A and 6, if the straight electrode pattern is used, the rubbing direction should be inclined with a certain angle to the straight electrodes such that a rubbing direction RD2 is applied to the upper substrate and an opposite rubbing direction RD1 is applied to the lower substrate. Moreover, as shown in FIGS. 5B and 6, if the zigzag-shaped electrodes are adopted, the rubbing direction should be parallel with the data lines DL, for example, 90 or 270 degrees such that a rubbing direction RD'2 applied to the upper substrate is opposite to the rubbing direction RD'1 applied to the lower substrate.
On the bare glass 99, it is possible to arrange different-sized liquid crystal cells VIa and VIb, for example, 30 inches and 15 inches. When forming the four 30 inches liquid crystal cells VIa and the three 15 inches liquid crystal cells VIb as shown in FIG. 6, widths W1 and W2 of those liquid crystal cells VIa and VIb should be arranged in the same direction because of the rubbing direction applied to the bare glass. Namely, the liquid crystal cells VIa and VIb let the widths W1 and W2 be arranged horizontally. If the 15 inches liquid crystal cells VIb are disposed to let the width W2 be arranged vertically, those 15 inches liquid crystal cells VIb may malfunction because the rubbing direction applied to the bare glass is substantially perpendicular to the electrodes unlike the rubbing direction shown in FIGS. 5A and 5B. Accordingly, there are some limitations in arranging the liquid crystal cells on the bare glass.